FYI, here's an updated version of my patch to not run pure flushes
that also works on SCSI/ATA and not just virtio. I suspect the patch
alone might not be enough, but together with a variant of Neil's
suggestion might do the trick, with my patch taking care of the
highend-devices and Neil's scheme of taking care of stupid SATA disks.
Index: linux-2.6/block/blk-flush.c
===================================================================
--- linux-2.6.orig/block/blk-flush.c 2010-11-30 17:27:33.108254088 +0100
+++ linux-2.6/block/blk-flush.c 2010-11-30 17:27:38.790004333 +0100
@@ -143,6 +143,17 @@ struct request *blk_do_flush(struct requ
unsigned skip = 0;
/*
+ * Just issue pure flushes directly.
+ */
+ if (!blk_rq_sectors(rq)) {
+ if (!do_preflush) {
+ __blk_end_request_all(rq, 0);
+ return NULL;
+ }
+ return rq;
+ }
+
+ /*
* Special case. If there's data but flush is not necessary,
* the request can be issued directly.
*
Index: linux-2.6/drivers/scsi/scsi_lib.c
===================================================================
--- linux-2.6.orig/drivers/scsi/scsi_lib.c 2010-11-30 17:27:33.120254298 +0100
+++ linux-2.6/drivers/scsi/scsi_lib.c 2010-11-30 17:27:38.791003634 +0100
@@ -1060,7 +1060,7 @@ int scsi_setup_blk_pc_cmnd(struct scsi_d
* that does not transfer data, in which case they may optionally
* submit a request without an attached bio.
*/
- if (req->bio) {
+ if (req->bio && !(req->cmd_flags & REQ_FLUSH)) {
int ret;
BUG_ON(!req->nr_phys_segments);
--
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